The use of high-frequency electromagnetic energy as a means of heating polymers is an advancing art which finds application in many fields. For instance, sealing of a polymer to itself or to some other substrate can be an important commercial technique in producing a desired end-product. However, there are some polymers which are not generally suitable for high-frequency heating operations. In such polymers, high-frequency heating either does not occur, or if it occurs, it does so slowly. In production assembly lines, a quick heat-seal is important.
Among the polymers which are ill-suited for high frequency heating are olefin polymers, e.g., polymers and copolymers of ethylene, propylene, styrene or other .alpha.-olefinically unsaturated hydrocarbons having 2 to 10 carbon atoms. Some of these olefin polymers may contain polar groups, or polarizable groups which are generally introduced by the copolymerization of acrylic (or methacrylic) acids or their alkyl esters as comonomers with the .alpha.-olefins Such polar groups may, at high levels of incorporation, tend to impart a modicum of high-frequency heatability to the polymer, but the efficacy is generally so slight that it is not feasible on a commercial basis. Some polymers having polar groups, e.g., chlorinated polyethylene (CPE), ethylene/vinyl acetate copolymer (EVA), polyvinylchloride (PVC), polyvinylidene chloride (PVDC), and polyamides can be radiation heated at certain frequencies of electromagnetic radiation, but are not generally heated at the higher frequencies which are frequencies of current commercial interest.
Polyethylene is shown in the art to be substantially unsuitable for high-frequency heating unless sensitizers are added to the polymer; this observation applies regardless of whether the polyethylene is linear or branched, or regardless of whether it is low, medium, or high density (see, for example, U.S. Pat. Nos. 3,336,173; 3,640,913; and 3,810,799).
U.S. Pat. Nos. 4,847,155; 4,787,194; 4,766,035; 4,762,731; 4,684,576; 4,678,713; 4,671,982; 4,660,354; 4,640,865, 4,601,948; and 4,600,614, all to Lancaster et al., disclose high frequency (HF) heatable or sealable ethylene-carbon monoxide (ECO) copolymers. It is stated, for example, in U.S. Pat. No. 4,601,948 that non-HF-heatable or non-HF-sealable polymers can be made to be HF-sealable by either incorporation of carbon monoxide by copolymerization or by blending or grafting a carbon monoxide copolymer or terpolymer into the polymer matrix. This patent additionally discloses ethylene-carbon monoxide-acrylic acid and ethylene-carbon monoxide methacrylic acid terpolymers which have HF-sealability and improved adhesion.
Grafting unsaturated monomer molecules onto .alpha.-olefin polymers and copolymers has been disclosed in a number of patents. The grafting technique has been used to modify the polymer properties to which the grafted molecules are attached. The following patents are representative of the prior art on grafting: U.S. Pat. Nos. 2,970,129; 3,177,269; 3,270,090; 3,873,643; 3,882,194; 3,886,227; 4,087,587; 4,087,588; 4,239,830; 4,298,712; 4,394,485; U. K. Patent 2,081,723; Jap.
Kokai 49(1973)-129742.
U.S. Pat. No. 4,684,576 to Tabor et al. discloses maleic anhydride grafted high density polyethylene (HDPE) which is blended with low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). These blends are stated to impart enhanced adhesive properties. These grafted and blended polymers lack carbon monoxide as a component, however, and are not suitable to be heated by high frequency electromagnetic energy.
From U.S. Pat. No. 4,814,378 to Lutz, it is known to blend ECO with copolymers of a vinyl aromatic compound and an unsaturated cyclic anhydride compound. Blends of ECO with styrene-maleic anhydride copolymer, for example, are stated to have improved processability and melt stability.
U.S. Pat. Nos. 4,391,923 to Rys-Sikora; and 4,613,533 to Loomis et al. disclose blends of ethylene/X/Y formula ethylene terpolymers with polyvinyl halides such as PVC, which are used to make molded or low density, closed-cell, foamed articles. The Y component is stated to be CO or SO.sub.2 and the X component is a softening monomer. U.S. Pat. No. 4,480,054 to Enderle is similar except that an ethylene-methacrylic acid (EMAA) or ethylene-acrylic-acid (EAA) copolymer is blended with ethylene-vinyl acetate-carbon monoxide terpolymer (EVACO) in place of PVC.
From U.S. Pat. No. 4,841,020 to Drent, it is known to prepare terpolymers of carbon monoxide, a functionally substituted ethylenically unsaturated compound wherein the functional substituent, which may be a carboxylic acid, is separated from the ethylenic unsaturation by a divalent bridging group of at least one carbon, and optionally an ethylenically unsaturated hydrocarbon.